The present invention relates to electromechanical transducers, i.e. linear electric motors and linear electric generators. In particular, the invention relates to the electric motors which are used in linear compressors of refrigerators or coolers, for instance those used to cool radio transmission antennae, and to generators, such as those driven by Stirling engines.
The linear electric transducer of the present invention may be used as a motor in valveless compressors, particularly for driving stirling cycle coolers or pulse tube coolers, valved compressors, for instance domestic and industrial “Freon” type refrigerators, Gifford McMahon (GM) coolers and oil-free gas compressors, pumps in clean circulation systems, such as for medical purposes.
Linear motors previously used in these applications are not standard components that are then integrated with other components. In general they are custom designed. Because of the wide range of applications requiring linear motors there is already a range of motor designs tailored to particular requirements. These include moving coil, moving magnet and moving iron designs. Despite the range of existing motor designs there is a lack of a design that is truly suitable for large-scale low cost manufacture. This invention seeks to fill this gap.
Moving coil linear electric motors of the prior art require flexible current leads and a large amount of magnet material resulting in high cost of manufacture.
Moving magnet and moving iron linear electric motors of the prior art typically consist of a number of magnetic circuits formed by annular cores each having an air gap and an electric coil for creating a magnetic flux in the air gap. The air gaps are aligned along a direction of movement of an armature received into the air gaps. The armature may be iron or a permanent magnet or electromagnet. Such motors, though, generally suffer from a lack of robustness or complicated construction that is not very compatible with other aspects of linear machine technology.
The invention relates in particular to the geometry of the components of the electromechanical transducer, in particular cores for the coils which allow the magnetic circuits of the transducer to be closely positioned along the direction of movement of the armature.
According to the present invention there is provided an electromechanical transducer comprising:                a first core providing a first flux path and defining a first air gap;        at least one first stationary coil disposed around a portion of said first core;        a second core providing a second flux path and defining a second air gap;        at least one second stationary coil disposed around a portion of said second core; and        a first armature arranged for linear motion along a longitudinal axis through said first and second air gaps; and        wherein        said first and second air gaps are positioned adjacent one another in spaced apart relationship such that said longitudinal axis passes through said first and second air gaps; and        said at least one first stationary coil and said at least one second stationary coil are positioned on opposite sides of a longitudinal plane in which said longitudinal axis lies.        
There are several advantages derived from the positioning of the first and second stationary coils on opposite sides of a longitudinal plane in which the longitudinal axis of the armature lies. This geometry leads to the advantage that because the bulky first and second stationary coils can at least partly overlap in the direction of the longitudinal axis of the motor, the first and second air gaps can be positioned close together. The positioning of the air gaps close together in the longitudinal axis results in a more efficient use of the armature, lower moving mass and a more compact design. The provision of coils around first and second cores means that for permanent magnet and moving iron designs the armature does not need coils and flexible current leads are not required. The magnetic circuits in this invention have minimal unwanted air gaps and make more efficient use of magnet material than many linear electric motors of the prior art. Finally, the construction of such an electromechanical transducer is simpler than many of the electromechanical transducers of the prior art.
The flux paths in the cores are in a single plane (i.e. planar geometry) and thus it is possible to manufacture the cores of laminations which can easily be stamped out of sheet metal.
Preferably the first and second cores are positioned such that the first and second flux paths are substantially on opposite sides of the longitudinal plane.
In this way it is possible to further reduce the size of the electromechanical transducer, to minimise the distance between the first and second air gaps and maximise the shape of the cores for efficiency. Furthermore, with the cores positioned in that way the size of the first and second coils is not constrained by the need to keep the distance between the first and second air gaps small.
The armature may have a substantially rectangular cross-section so that the armature is particularly simple to construct offering lower cost of manufacture. If the armature is made of a permanent magnet, such geometry makes it easy to magnetise the armature in the correct direction. Alternatively, the armature may comprise at least one electromagnet with one or more coils and soft iron cores. Flexible current leads would be required to take current into the coils.
If the at least one first stationary coil comprises two stationary coils and the at least one second stationary coil comprises two stationary coils, both the first stationary coils and the second stationary coils can be arranged to give good use of space within a cylindrical housing.
Preferably the armature comprises a permanent magnet so the need for an armature comprising coils and the required flexible current leads is eliminated.
Advantageously the electromechanical transducer is a variable reluctance electric motor and the armature is comprised of a soft high magnetic permeability material and thus the need for expensive permanent magnetic material is eliminated.
The electromechanical transducer of the present invention may further comprise a third core for providing a third flux path and for defining a third air gap wherein the third air gap is positioned adjacent to the second air gap in spaced apart relationship and such that the longitudinal axis passes through the third air gap. In such a case the at least one first stationary coil may be disposed around a portion of the third core for generating a magnetic field across the third air gap.
Such an arrangement can be beneficial in that the length of travel of the armature in the linear electric motor may be increased.
Furthermore, the electromechanical transducer may comprise a fourth core for providing a fourth flux path and for defining a fourth air gap wherein the fourth air gap is positioned adjacent to the third air gap in spaced apart relationship and such that the longitudinal axis passes through said fourth air gap. In such a case the at least one second stationary coil may be disposed around a portion of the fourth core for generating a magnetic field across the fourth air gap and thus the length of travel of the armature of the electromechanical transducer may be still further be increased.
Alternatively with additional armature components the transducer power can be increased without increasing the transducer diameter.
Preferably the armature comprises a carriage comprising the first armature and a second armature separated by approximately the length of the second air gap in the direction of the longitudinal axis. This increases the total change in flux linkage and hence increases the power handling capacity. This increase in capacity can be achieved without increasing the motor diameter, the number of coils or the complexity of the control systems that provide current to the coils.
The first and second armatures may be permanent or electro-magnets or made of a soft high magnetically permeable material, the carriage may comprise a third armature which is a magnet, the third armature being polarised in the opposite direction to the first and second armatures and positioned between the first and second armatures. This further increases the power handling capacity of the linear electric transducer.
Of course any number of cores may be provided all connected in the same way as the first, second, third and fourth cores as described above.
The electromechanical transducer may be arranged to operate as a linear electric motor wherein said at least one first and second stationary coils are for generating magnetic fields across said first and second air gaps respectively and said first armature is arranged for linear motion in response to said generated magnetic fields.
The present invention also provides a linear compressor comprising such a linear electric motor.
The present invention also provides a refrigerator comprising such a linear compressor.
The electromechanical transducer may also be arranged to operate as a linear electric generator.